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Al2O3 catalysts
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Promotion of CO2 methanation activity and CH4 selectivity at low temperatures over Ru/CeO2/Al2O3 catalysts Shohei Tada a,b, Ochieng James Ochieng a,1, Ryuji Kikuchi a,*, Takahide Haneda c, Hiromichi Kameyama c a
Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Japan Society for the Promotion of Science, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 103-0083, Japan c Tokyo Gas Co., Ltd., 1-7-7 Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa 230-0045, Japan
article info
abstract
Article history:
The effect of CeO2 loading amount of Ru/CeO2/Al2O3 on CO2 methanation activity and CH4
Received 27 December 2013
selectivity was studied. The CO2 reaction rate was increased by adding CeO2 to Ru/Al2O3,
Received in revised form
and the order of CO2 reaction rate at 250 C is Ru/30%CeO2/Al2O3 > Ru/60%CeO2/Al2O3 > Ru/
17 April 2014
CeO2 > Ru/Al2O3. With a decrease in CeO2 loading of Ru/CeO2/Al2O3 from 98% to 30%,
Accepted 20 April 2014
partial reduction of CeO2 surface was promoted and the specific surface area was enlarged.
Available online 16 May 2014
Furthermore, it was observed using FTIR technique that intermediates of CO2 methanation, such as formate and carbonate species, reacted with H2 faster over Ru/30%CeO2/Al2O3 and
Keywords:
Ru/CeO2 than over Ru/Al2O3. These could result in the high CO2 reaction rate over CeO2-
Ruthenium
containing catalysts. As for the selectivity to CH4, Ru/30%CeO2/Al2O3 exhibited high CH4
Hydrogen transfer
selectivity compared with Ru/CeO2, due to prompt CO conversion into CH4 over Ru/30%
Ceria
CeO2/Al2O3.
CO2 methanation
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Utilization of green hydrogen, produced by renewable energy (such as biomass, solar, and wind energy), has been urged worldwide in order to decrease the reliance on fossil fuel, to minimize the environmental pollution, and to produce hydrogen sustainably [1]. Fuel cells have been developed as a
promising power generator with high efficiency and low environmental impact. Fuel cells directly convert chemical energy of hydrogen into electrical energy and thus avoid the intermediate steps of producing heat and mechanical power works typical of conventional power generation methods. Thus the efficiency of the energy conversion from fuels is not limited by thermodynamic limitations of heat engines such as Carnot efficiency. In addition, the fuel cells can produce
* Corresponding author. Tel./fax: þ81 3 5841 1167. E-mail addresses: [email protected], [email protected] (R. Kikuchi). 1
Present address: Department of Environment Systems, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan. http://dx.doi.org/10.1016/j.ijhydene.2014.04.133 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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electricity (and heat) with low pollutant emissions due to the avoidance of combustion. Hydrogen is a gas at standard temperature and pressure, and has many difficulties in storing and transporting hydrogen. In order to overcome the difficulties, many studies have been conducted on the following hydrogen carriers: CH4 [2e5], NH3 [6], MeOH [5,6], dimethyl ether [5,7], formic acid [5,8], and organic chemical hydrides [1,9,10]. Especially CH4, made from CO2 methanation (CO2 þ 4H2 / CH4 þ 2H2O), has become a promising candidate as hydrogen source to be used in fuel processors. The advantage of CH4 as a hydrogen carrier is non-toxicity in contrast to ammonia and methanol, and has a high energy density (3 times higher volumetric energy density than hydrogen). Moreover the well-developed infrastructures of natural gas can be adopted for methane, resulting in the easy storage and transportation of methane [6]. Added to this, enormous carbon dioxide emitted into atmosphere can be recycled as the feedstock of CO2 methanation, leading to the solutions to global warming [2]. If the cost of green hydrogen production can be lowered, this attempt will succeed admirably. CO is one of the intermediates in CO2 methanation and produced in the following reactions: reverse water gas shift reaction (RWGS reaction, CO2 þ H2 / CO þ H2O) at more than 200 C [11e13] and CO2 decomposition (CO2ads / COads þ Oads) at less than 100 C [3,14,15]. The CO2 reduction to CO is considered to be a ratedetermining step of CO2 methanation. Endothermic RWGS reaction is favored thermodynamically and kinetically at high temperatures. The successive CO methanation (CO þ 3H2 / CH4 þ H2O) is, however, of exothermic nature, and thus lower reaction temperature is desirable for higher CH4 selectivity. It is, therefore, important to enhance the reaction rate of RWGS reaction and CO methanation at low temperatures. The mechanism of CO2 methanation has been mainly investigated using Ni [2,11,16e18] and noble metal (such as Ru [12], Rh [3,14,15], and Pd [13]) as active species supported on various metal oxides. As for carbon dioxide methanation over supported metal catalysts, it is well accepted that carbon dioxide adsorbed on the catalyst surface reacts with hydrogen on the metal to produce methane [11,16,19]. However, there still exists argument that the active sites of the conversion from CO2 to CO over supported catalysts are the metal surface [17,20,21] or interface between the metal and the support [12,22e24]. The reported catalysts for CO2 methanation were summarized in Table 1. Sharma et al. reported that over Ru-doped ceria, Ce0.95Ru0.05O2, prepared by a combustion method, CO is not a reaction intermediate of CO2 methanation [25]. The catalyst showed higher catalytic activity for CO2 methanation
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than 5 wt% Ru/CeO2, and CO2 conversion and CH4 selectivity were 55% and 99%, respectively, at 450 C by feeding 13% CO2, 54% H2, and 33% Ar at GHSV ¼ ca. 10,000 h1. In addition, CH4 was not produced up to 550 C when CO and H2 were fed to the catalyst. Ocampo et al. investigated CO2 methanation over Ni/ Ce0.72Zr0.28O2 catalysts [19]. Nickel sintering on this catalysts was restricted by the incorporation of nickel cations into the fluorite-structured Ce0.72Zr0.28O2, resulting in high stability for 150 h at 350 C during CO2 methanation (H2/CO2/N2 ¼ 36/9/10, GHSV ¼ 43,000 h1). In our previous work, the catalytic performance of Ni/CeO2, Ni/Al2O3, Ni/MgO, and Ni/TiO2 was investigated for CO2 methanation [26]. It is concluded that CeO2 plays an important role in promoting CO2 methanation and enhancing CH4 selectivity. Sufficient amount of CO2 adsorbed on CeO2 which is known as a basic oxide, and CO2 was reduced easily due to the oxygen vacancies on CeO2 support, leading to high CO2 conversion at low temperatures below 350 C. In addition, methanation of CO, which is the intermediate of CO2 methanation, was improved over Ni/ CeO2, resulting in high CH4 selectivity at low temperatures. Over CeO2-containing catalysts, CeO2 surface with surface oxygen vacancies influences on CO2 methanation activity. Thus an effective utilization of CeO2 by enlarging the surface area is anticipated to improve the CO2 methanation. We focused on ceria-supported materials as support material, and tried to increase CeO2 surface area. The ceria-supported materials have been reported in the following fields: cellulose gasification (Rh/CeO2/SiO2) [27], HCl oxidation to Cl2 (CeO2/ ZrO2) [28], oxidation of 1,2-dichloroethane (CeO2/USY zeolite) [29], and CO methanation (CeO2-doped Ni/Al2O3) [30]. Rynkowski et al. studied Ru/CeO2eAl2O3 catalysts for CO2 methanation, and observed CeAlO3 formed by high temperature reduction pretreatment (above 973 K) [31]. They expected that the stabilization of Ce3þ in CeAlO3 leads to high activity of CO2 methanation. Trovarelli et al. improved CeO2 reducibility on Rh/CeO2/SiO2 due to deposition of CeO2 on silica and H2 spillover from Rh to CeO2, leading to high CO2 methanation activity [32]. Therefore, these two groups suggested the importance of partially-reduced CeO2 surface on CO2 methanation. In this study, Ru species was loaded on CeO2/Al2O3, CeO2, and Al2O3 by an impregnation method, and the effect of CeO2 loading amounts on CO2 methanation activity and CH4 selectivity was investigated using these supported Ru catalysts. Expansion of CeO2 surface area by dispersing CeO2 on Al2O3 is expected to increase (i) CO2 adsorption per unit catalyst weight on the CeO2 catalysts, and (ii) the number of utilizable oxygen vacancies, resulting in high CO2 conversion at low temperatures. Furthermore CeO2 supported metal catalysts are anticipated to show high CO methanation activity, leading to high CH4 selectivity at low temperatures.
Table 1 e Reported CO2 methanation catalysts. Catalysts
Gas composition
Grac¸a et al. Ocumpo et al.
14%Ni7%CeUSY 5%Ni/Ce0.72Zr0.28O2
CO2/H2/N2 ¼ 9/36/10 CO2/H2/N2 ¼ 9/36/10
Sharma et al. Tada et al.
Ce0.95Ru0.05O2 10%Ni/CeO2
CO2/H2/Ar ¼ 13/54/33 CO2/H2 ¼ 1/4
GHSV/h1 43,000 21,000 43,000 10,000 10,000
CO2 conversion
68%, 400 C ca. 80%, 350 C ca. 65%, 350 C 55%, 450 C 87%, 300 C
Reference [18] [19] [25] [26]
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Experimental methods Catalyst preparation Chemicals Chemicals used in the synthesis were Al2O3 (JRC-ALO-6) provided by Catalysis Society of Japan, cerium (III) nitrate hexahydrate (Ce(NO3)3$6H2O, Aldrich, 98%), and ruthenium nitrate solution (Ru(NO3)3, Tanaka Kikinzoku Kogyo K.K., 3.920 wt%).
Preparation of CeO2/Al2O3 and CeO2 CeO2/Al2O3 was prepared by an impregnation method. Al2O3 was impregnated with a 100 ml aqueous solution of Ce(NO3)3$6H2O. The samples were dried at 100 C and then calcined at 500 C for 3 h in air. The CeO2 loadings were 30 and 60 wt%. Pure CeO2 was also made in the following manner: drying of Ce(NO3)3$6H2O solution at 100 C, and calcination at 500 C for 3 h in air. The samples of Al2O3, 30%CeO2/Al2O3, 60%CeO2/Al2O3, and CeO2 were hereafter abbreviated as AL, 30CEAL, 60CEAL, and CE, respectively.
Preparation of supported Ru catalysts Supported Ru catalysts were prepared by an impregnation method. The AL, 30CEAL, 60CEAL, CE were impregnated with a 100 ml aqueous solution of Ru(NO3)3. All samples were dried at 100 C and then calcined at 500 C for 3 h in air. The Ru loading was 2 wt%.
Characterization The specific surface area of each catalyst was evaluated by the BET method using nitrogen adsorption (Micromeritics, ASAP2000). Before the measurement, all samples were dried in vacuum at 150 C. Transmission electron microscopy (TEM) monitored the morphology of the CeO2 and Al2O3 particles on the catalysts (JEOL, JEM-2000EX, acceleration voltage 200 kV). The crystalline phase of catalysts was determined by X-ray diffraction (XRD) using Rigaku, RINT 2400 (voltage 40 kV; current 100 mA). X-ray photoelectron (XP) spectra were measured by a JEOL JPS-90SX with MgKa radiation. The binding energy for each XP spectroscopy measurement was referenced to the C 1s peak (285.0 eV). Temperature programmed desorption of CO2 (CO2-TPD) was carried out in a flow system (Quanta Chrome, CHEMBET-3000). About 100 mg of each catalyst was flushed at 500 C for 30 min in He flow and cooled to 100 C in He flow. A gaseous mixture of 10% CO2/He was fed to the reactor at 100 C for 30 min, and the temperature was raised at a constant heating rate of 10 C min1 under He flow. The reducibility of ruthenium and cerium was evaluated by temperature programmed reduction (TPR) in a H2 flow system (Quanta Chrome, CHEMBET-3000). In a gas stream of 5%H2/Ar, about 20 mg of the samples was heated up to 400 C at 10 C min1. Before the TPR measurement, each sample was flushed with He flow at 500 C for 30 min and cooled to room temperature. Ru and Ce loading amounts on each catalyst were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, SII NanoTechnology, SPS4000).
Fourier transform infrared spectroscopy measurement A Fourier transform infrared spectroscopy (FTIR) cell equipped with high purity CaF2 windows and capability for heating and cooling was placed in a JASCO FTIR 6100 instrument with a mercury cadmium telluride (MCT) detector (JASCO, MCT6000M). The powdered samples were pressed into a thin self-supporting disk and set in the cell. Typically, 50 scans were collected for one spectrum, and the results are presented as absorbance spectra. The following two experiments were carried out.
Temperature programmed desorption of CO2 with FTIR Adsorption and desorption characteristics of CO2 on Ru/CeO2/ Al2O3 were investigated. The disk was flushed with N2 stream at 300 C as a pretreatment. Temperature was then decreased in a stepwise mode to room temperature and background spectra were attained at desired temperatures in interest after equilibration for 10 min-on-stream. The sample was exposed to the mixed gas (10%CO2/He) for 10 min at room temperature. The cell was purged with N2 and their spectra were recorded at the desired temperatures during heating (heating rate ¼ 5 C/min).
Reaction intermediate observation during CO2 methanation on CeO2-containg catalysts After reduction by 5%H2/Ar at 350 C for 30 min in the cell, the samples were held at 200 C in a N2 stream. Then background spectra were collected at 200 C. The samples were exposed to the reaction gas (CO2/H2 ¼ 1/9) for 5 min at 200 C, and then their spectra were recorded 10 min after the mixed gas had been removed by flushing with N2. Subsequently, the behavior of products adsorbed on the catalyst surface was also examined in N2 and H2/Ar gas. After 5 min reaction, the samples were exposed to N2 or 5%H2/Ar at 200 C, and then their spectra were measured.
Activity tests Catalytic performance of CO2 methanation was evaluated in a 4mm I.D. fixed-bed quartz tubular reactor at atmospheric pressure. The reaction temperature was measured at the inlet of the catalyst bed by a K-type thermocouple. Catalyst powder of 300 mg was set in the reactor, and then reduced at 500 C for 30 min in 5% H2/Ar flow prior to the run. A gaseous mixture of 20% CO2/H2 was fed at a space velocity of 10,000 h1. Catalytic activity of CO methanation was also measured in the same apparatus by using a sample weight (300 mg) and a reaction gas condition (1%CO/H2, 10,000 h1). The gas composition at the reactor outlet was analyzed with a micro gas chromatograph (Varian, CP-4900) equipped with MS5A, COX, and PPQ columns and a thermal conductivity detector (TCD). The outlet gas concentrations were described in a basis of the dry gas composition.
Results and discussion Effect of CeO2 loading on catalytic activity over Ru/CeO2/ Al2O3 Fig. 1(a) shows CO2 conversion over the supported Ru catalysts as a function of reaction temperature. CO2 conversion, CH4 selectivity, and CO2 reaction rate are defined as follows:
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upper shift of the measured CO2 conversion to equilibrium one in Fig. 1(a). In Fig. 1(b) the CH4 selectivity over the supported Ru catalysts is plotted against reaction temperature. The selectivities of Ru/AL, Ru/30CEAL, and Ru/60CEAL were close to 1 at low temperatures below 450 C. The selectivity of Ru/CE was 0.986 at 250 C, and ca. 1 at temperatures between 300 and 450 C. The rest of converted CO2 existed as CO, which means that the improvement of CO methanation activity leads to high CH4 selectivity in CO2 methanation. As for Ru/ 30CEAL and Ru/60CEAL CO methanation activity was expected to be high compared to Ru/CE because of the high CO2 reaction rates and CH4 selectivities. The conversion curves of CO during CO methanation over the supported Ru catalysts were shown in Fig. 2. The concentration of CO over Ru/30CEAL increased rapidly compared to that over Ru/CE, which indicates that Ru/30CEAL showed higher CO methanation activity than Ru/CE, and thus Ru/ 30CEAL was concluded to have a high CH4 selectivity during CO2 methanation.
Characterization XRD Fig. 1 e (a) CO2 conversion and (b) CH4 selectivity over supported Ru catalysts. Reaction conditions: 20 vol% CO2, 80 vol% H2. GHSV [ 10,000 hL1. The temperature was measured at the catalyst bed inlet.
FCO2 ;out FCO2 ;in
(1)
FCH4 ;out FCO2 ;in FCO2 ;out
(2)
CO2 conversion ¼ 1
CH4 selectivity ¼
CO2 reaction rate ¼
CO2 conversion FCO2 ;in W
(3)
where FCO2 ; in and FCO2 ; out are molar flow rates of CO2 at the reactor inlet and outlet, respectively, FCH4 ; out is the outlet molar flow rate of CH4, and W is the sample weight. Over Ru/ AL CO2 conversion was gradually increased as the temperature was raised, and at 500 C reached the equilibrium one represented by the solid line in Fig. 1(a) calculated by taking CO2 methanation and RWGS reaction into account. Over the other supported Ru catalysts the conversion was rapidly increased consistently with increasing temperature up to 350 C, and then achieved the equilibrium one. The reaction rates at 250 C over Ru/AL, Ru/30CEAL, Ru/60CEAL, and Ru/CE were 1.2, 6.2, 4.9, and 4.6 ml min1 g1 cat, respectively, which indicates that Ru/CeO2/Al2O3 catalysts (especially Ru/30CEAL) have higher CO2 methanation activity than Ru/Al2O3 and Ru/ CeO2. As for CeO2-containing catalysts, CO2 conversion above 400 C was slightly above the equilibrium one probably due to temperature gradient in the catalyst bed. Actually, CO2 conversion at 450 C was in the order of the length of the catalyst bed: Ru/30CEAL (31 mm) > Ru/60CEAL (22 mm) > Ru/CE (16 mm). According to the gradient, part of CO2 was methanated at the temperature measured by K-type thermocouple, and the rest at lower temperature, probably leading to the
To investigate the crystalline phases of supported Ru catalysts, powder XRD measurements of AL, CE, Ru/AL, Ru/ 30CEAL, Ru/60CEAL, and Ru/CE were performed, and all peaks were identified with JCPDS files, as illustrated in Fig. 3. Both AL and CEAL catalysts exhibited similar diffraction patterns with peaks at 38, 39, 46, and 67 , which correspond to (311), (222), (400), and (440) planes of g-Al2O3, respectively. The supported Ru catalysts have 4 peaks at 28, 35, 54, and 58 in the XRD patterns, which correspond to (110), (101), (211), and (220) planes of RuO2, respectively. Both CE and CEAL catalysts showed the patterns with the peaks at 29, 33, 47, 56, 59, and 69 , which correspond to (111), (200), (220), (311), (222), and (400) planes of fluorite-structured CeO2, respectively. The mean crystallite size of RuO2 and CeO2 was estimated from the diffraction peak of RuO2 (101) and CeO2 (111) plane by Scherrer equation, D¼
Kl b cos q
(4)
where K is the shape factor (0.89), l is X-ray wavelength (0.154 nm), b is the line broadening at half the maximum
Fig. 2 e CO conversion over Ru/CE and Ru/30CEAL. Reaction conditions: 1 vol% CO, 99 vol% H2. GHSV [ 10,000 hL1.
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Fig. 3 e XRD patterns of as-prepared AL, Ru/AL, Ru/30CEAL, Ru/60CEAL, Ru/CE, and CE. JCPDS-card number: fluoritestructured CeO2 (71-4199), g-Al2O3 (29-0063), and RuO2 (40-1290).
intensity in radians, and q is Bragg angle [33]. The sizes of RuO2 on Al2O3, 60%CeO2/Al2O3, and CeO2, as summarized in Table 2, were ca. 30 nm, and that on 30%CeO2/Al2O3 was 16 nm. The sizes of CeO2 were almost constant (ca. 8 nm).
Fig. 4(d) exhibits that as for 30CEAL small CeO2 particles (average 6.5 nm) were dispersed on rod-shaped g-Al2O3, and the formation of the CeO2 secondary particles (ca. 50 nm, indicated as solid arrows) can be observed.
Electron microscopic image
Reducibility and loading amount of supported Ru catalysts measured by ICP and H2-TPR
As summarized in Fig. 4, transmission electron microscopy (TEM) was used to examine the AL, CE, 60CEAL, and 30CEAL. Fig. 4(a) displays that the shape of g-Al2O3 was rod, and the diameter was ca. 3 nm. Fig. 4(b) shows a lot of CeO2 particles with average size of 13 nm. On the CeO2 particles, slightly bright dots can be seen, which means that this CeO2 had a porous structure. Fig. 4(c) shows that 60CEAL is composed of a lot of CeO2 particles and small amount of rod-like g-Al2O3 (indicated as solid arrows). The average particle size of CeO2 on 60CEAL was 10 nm and a little smaller than that on CE.
Fig. 5 shows the H2-TPR profiles of the supported Ru catalysts. Over the 4 catalysts, reduction peaks were detected below 300 C, which have been ascribed to reduction of RuO2 [34e36]. H2 consumption was calculated by assuming reduction of Ru4þ (RuO2) to Ru0, as summarized in Table 3. As for Ru/AL, the consumption amount estimated by H2-TPR 1 was 0.29 mmol g1 cat. This value is close to 0.30 mmol gcat which accords with the H2 consumption by RuO2 reduction to 1.5 wt% Ru metal. Thus Ru oxides on Ru/AL was completely
Table 2 e Characterization results for support materials and supported Ru catalysts. Catalysts
CE þRu 60CEAL þRu 30CEAL þRu AL þRu a b
SSA/m2 g1
69 73 81 74 104 115 164 162
Pore volume/cm3 g1
0.20 0.18 0.25 0.22 0.41 0.42 0.66 0.62
Crystallite sizea/nm RuO2(101)
CeO2(111)
e 28 e 28 e 16 e 30
9.6 7.8 8.2 7.9 8.5 8.4 e e
Estimated from XRD patterns. CO2 adsorption per unit specific surface area. Estimated from CO2-TPD and N2 adsorption.
CO2 adsorptionb/(mmol g1)/(m2 g1)
0.72 1.40 0.40 0.32
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Fig. 4 e TEM images of (a) AL, (b) CE, (c) 60CEAL, and (d) 30CEAL. Particle size distributions of CeO2 in (e) CE, (f) 60CEAL, and (g) 30CEAL measured by the TEM observation.
reduced. On the other hand, the amounts of Ru on Ru/CE, Ru/ 60CEAL, and Ru/30CEAL by H2-TPR (0.67e0.77 mmol g1 cat) are almost twice as much those estimated by ICP-AES (0.32e0.35), which means that not only RuO2 but also a part of CeO2 was reduced below 300 C. Indeed CeO2 alone can be reduced, as evidenced by the broad peak at 470 C on the TPR profile [26], and the CeO2 reducibility was enhanced due to the presence of Ru. As for supported metal catalysts a part of H2 molecules dissociated on noble metals such as Ru may migrate to nearby metal oxides under a H2-rich condition. This phenomenon is known as a hydrogen spillover effect [13,36,37]. Thus it is expected that over our Ru/CE, Ru/60CEAL, and Ru/30CEAL CeO2 in the vicinity of Ru species was selectively reduced by spillover hydrogen. Interestingly, H2 consumption by CeO2 reduction per unit CeO2 weight was increased with decreasing CeO2 loadings, as listed in Table 3. This result indicates that the CeO2 particle size became smaller for the decrease in the CeO2 loadings, leading to enlarged utilizable CeO2 surface area.
Surface analysis of supported Ru catalyst BET surface areas and pore volumes of AL, 30CEAL, 60CEAL, and CE determined by N2 adsorption are listed in Table 2. The
Fig. 5 e H2-TPR profiles of Ru/AL, Ru/30CEAL, Ru/60CEAL, and Ru/CE (heating rate [ 10 C/min).
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Table 3 e Loading weight and H2 consumption. Catalysts
Ru/CE Ru/60CEAL Ru/30CEAL Ru/AL a b c
Loading weighta/wt%
H2 consumption/mmol g1 cat
Ru
CeO2
RuO2 reduction
1.8 1.8 1.6 1.5
98 52 26 e
0.35 0.35 0.32 0.30
a
H2 consumption by CeO2 reductionc/mmol gCeO2 1
b
Total 0.77 0.65 0.67 0.29
0.43 0.58 1.34 e
Calculated by ICP-AES. Estimated by the peak areas of H2-TPR spectra. The H2 consumption difference between total and RuO2 reduction per unit CeO2 weight.
surface area decreased from 164 to 69 m2 g1 with increase in CeO2 loading amounts, and the pore volume simultaneously dropped from 0.66 to 0.20 cm3 g1. Especially both the surface area and the pore volume of 60CEAL were similar to those of CE. This is because CeO2 plugged the surface and pores of Al2O3. The addition of Ru species to the supports did not change the surface area and the pore volume, indicating that the Ru components did not cause sintering and pore blockage of the support of the supports. XP spectra of Al 2p core electron levels for as-prepared Ru/ CE, Ru/60CEAL, Ru/30CEAL, and Ru/AL are shown in Fig. 6(a). The spectra from Ru/60CEAL, Ru/30CEAL, and Ru/AL have a single peak at 74 eV. This peak is attributed to Al3þ in g-Al2O3 [38]. The order of the intensity was Ru/AL S Ru/30CEAL > Ru/ 60CEAL, and there was no peaks of Al 2p over Ru/CE. Fig. 6(b) indicates O 1s spectra of the catalysts. The spectrum of Ru/CE shows a strong peak at 529 eV with a small shoulder at 531 eV. The first peak in the spectrum is attributable to a lattice oxygen in CeO2 and the second at 531 eV to a defect-oxygen and a
hydroxyl-like group in CeO2 [29,38,39]. The O 1s spectrum of Ru/60CEAL was split up into two peaks at 531 and 529 eV, which are assigned to lattice oxygen of Al2O3 and CeO2, respectively. As for Ru/30CEAL, the O 1s peak of Al2O3 with the binding energy at 531 eV was solely observed. In the Ru 3d region shown in Fig. 6(c), the peak at 281 eV appeared and belongs to Ru4þ in RuO2 [40]. The Ce 3d spectra for supported Ru catalysts are shown in Fig. 6(d). Six peaks in the spectra correspond to Ce4þ in CeO2 [38,39,41]. In order to discuss the amount of exposed Ce species, the surface atomic ratio RS is calculated from the following equation. RS ¼
ICe =SCe ICe =SCe þ IAl =SAl
(5)
, where Ix the intensity of peaks assignable to an element x in XP spectra, and Sx the atomic sensitive factor of x [42]. The molar ratio of Ce to Al on Ru/CeO2, Ru/Al2O3, and Ru/CeO2/ Al2O3 catalysts are summarized in Table 4. The bulk
Fig. 6 e XP spectra of (a) Al 2p, (b) O 1s, (c) Ru 3d, and (d) Ce 3d of supported Ru catalysts.
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Table 4 e Bulk and surface molar ratio of Ce and Al estimated from ICP-AES and XP spectra. Catalysts
Ru/CE Ru/60CEAL Ru/30CEAL Ru/AL a b
Molar ratio Ce/(Ce þ Al) Bulk RBa
Surface RSb
1 0.25 0.10 0
1 0.36 0.06 0
Calculated from ICP-AES results listed in Table 2. Estimated from the area of peaks attributed to Ce and Al.
composition ratios (RB) were initially approximated by the ICPAES results. With respect to Ru/30CEAL, RB was higher than RS, indicating that CeO2 tends to infiltrate into Al2O3 pore. This interpretation is also supported by N2 adsorption results described above e CeO2 particles sealed a part of Al2O3 pores. Concerning Ru/60CEAL, RB was lower than RS, which means that Al2O3 surface was covered with CeO2. In our previous study, the surface coverage by CO2-derived species, such as carbonate and formate species, on catalysts resulted in the high CO2 methanation activity [26]. Thus it is necessary to investigate CO2 adsorption on supported Ru catalysts. Fig. 7(a) shows a series of FTIR spectra of CO2 adsorbed on Ru/30CEAL recorded with increasing temperature. In the spectrum at 24 C, bands at 1650 and 1230 cm1 appeared, and were attributable to bidentate carbonate species [43]. Furthermore two peaks at 1570 and 1430 cm1 were assignable to unidentate carbonate species [43]. The intensity of the 4 peaks was decreased with temperature elevation from
Fig. 7 e (a) FTIR spectra of CO2 adsorbed on Ru/30CEAL in the course of desorption at elevated temperatures from 25 C to 300 C (heating rate: 5 C/min). (b) CO2-TPD profiles of Ru/AL, Ru/30CEAL, Ru/60CEAL, and Ru/CE (heating rate [ 10 C/min).
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24 C to 200 C, and then no peaks could be observed in the region between 2200 and 1100 cm1, which shows that CO2 was completely desorbed from Ru/30CEAL at 200 C. The TPD spectra obtained after CO2 adsorption on the supported Ru catalysts at 100 C are shown in Fig. 7(b). As for all catalysts, two desorption peaks were detected at around 110 C and 180 C. These are assignable to CO2 desorption because the peaks of carbonate species in FTIR spectra disappeared at the temperatures above 200 C (Fig. 7(a)). The amount of CO2 adsorption per unit catalyst weight was calculated by the peak area of CO2-TPD and BET surface area, as listed in Table 2. The order of the adsorption was Ru/ 60CEAL > Ru/CE > Ru/30CEAL > Ru/AL, indicating that a large amount of CO2 adsorbed on CeO2/Al2O3 supported Ru catalysts, especially Ru/60CEAL, compared with Ru/AL.
Reaction intermediates measured by FTIR analysis Fig. 8 shows the time variation of FTIR spectra at 200 C by feeding a model gas mixture of CO2/H2 ¼ 1/9 for CO2 methanation. The Ru/AL, Ru/30CEAL, and Ru/CE catalysts were reduced prior to the reaction. As for Ru/AL in Fig. 8(a), five distinct peaks at 1980, 1590, 1450, 1390, and 1370 cm1 became stronger as reaction time elapsed from 10 s to 5 min. For Ru/
Fig. 8 e FTIR spectra of (a) Ru/AL, (b) Ru/30CEAL, and (c) Ru/ CE during CO2 methanation with the model gas mixture (CO2/H2 [ 1/9) at 200 C. The spectra were obtained in the following manner: reaction for desired time, and flushing 10 min with N2.
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30CEAL in Fig. 8(b), the peaks at 2020, 1940, 1590, 1520, 1470, 1450, 1390, 1370, and 1290 cm1 arose during CO2 methanation reaction. On Ru/CE, right after the introduction of 10%CO2/H2 gas mixture into the IR cell, eight peaks at 2020, 1940, 1580, 1530, 1470, 1370, 1330, and 1290 cm1 were detected as shown in Fig. 8(c). On all catalysts, the bands at the wavenumbers between 2020 and 1940 cm1 were attributed to CO adsorbed linearly on Ru surface while the other bands to carbonate or formate species on the catalysts [24,44]. In order to assess the reaction steps in which these species are involved, transient experiments in H2/Ar atmosphere were undertaken. As shown in Fig. 9(a), (b), and (c), the bands at ca. 2000 cm1, attributed to CO adsorbed on Ru species, were diminished. Over Ru/CE, especially, a new peak at 1820 cm1 appeared with an increase in retention time under H2/Ar atmosphere, which is assigned to bridge-bonded CO on Ru [24]. As for Ru/AL in Fig. 9(a), the peaks at 1590, 1390, and 1370 cm1 were ascribed to formate species on Al2O3 [45]. In addition, the carbonate peak at 1450 cm1 was also unvaried. On Ru/CE (Fig. 9(c)), all peaks in the region 1600e1200 cm1 were weakened. To compare the area decreasing rate of each peak over Ru/CE, the spectra in Fig. 9(c) were divided into 6 peaks. The peak areas after desired time of reaction with H2/Ar were
Fig. 9 e FTIR spectra of (a) Ru/AL, (b) Ru/30CEAL, and (c) Ru/ CE after CO2 methanation for 5 min at 200 C. These spectra were recorded after a switch from the model gas mixture (CO2/H2 [ 1/9) to 5% H2/Ar.
defined as A, and the initial areas as A0. Fig. 10 shows the peak area ratios (A/A0) as a function of time-on-stream after switching to H2/Ar. The bands at 1580 and 1330 cm1 decreased at an identical rate, and are derived from disappearance of bidentate carbonate species on CeO2 [46,47]. Interestingly, the three curves for A/A0 at 1530, 1470, and 1370 cm1 were nearly coincident: the ratios of 1530, 1470, and 1370 cm1 decreased from 1 to 0.5 in 150 min. The peaks at 1530, and 1370 cm1 are ascribed to OeCeO asymmetric vibration and CeH in-plane bending vibration in formate species, respectively, while the band at 1470 cm1 assigned to monodentate carbonate [46]. Fig. 11 shows FTIR spectra of Ru/ CE recorded 1 h after a switch from the model gas mixture for CO2 methanation to N2. Under N2 atmosphere the carbonate peaks (1580 and 1330 cm1) gradually disappeared, while the formate peaks (1540 and 1390 cm1) were unchanged. Thus, what follows these results are: (i) the carbonate and formate species react with H2, (ii) the carbonate species on Ru/CE are decomposed or desorbed at high temperature such as 200 C, and (iii) the formate species on Ru/CE are thermally stable at 200 C. Over Ru/30CEAL, the intensities of the peaks attributed to RueCO (2020 and 1940 cm1), carbonate species (1330 cm1), and formate species (1590, 1390, 1370 cm1) were decreased with increasing reaction time under H2/Ar, as shown in Fig. 9(b). The peaks of CO adsorbed on Ru were rapidly diminished compared to Ru/CE. From this result and CO methanation activity test (Fig. 2), Ru/30CEAL exhibited high CO methanation activity, and thus high CH4 selectivity was attained in CO2 methanation. Interestingly the peak at 1390 cm1, assignable to formate species on Ru/Al2O3, remained almost unchanged even under H2/Ar flow. The peaks attributed to formate species on Ru/30CEAL and Ru/CE were weakened rapidly compared to Ru/AL, which indicates that the decomposition of formate species on CeO2-containg catalysts was promoted.
Fig. 10 e Decrease in FTIR peak area ratio to the initial peak area at the respective wavenumbers under 5% H2/Ar flow at 200 C as a function of reaction time.
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references
Fig. 11 e FTIR spectra of Ru/CE after CO2 methanation for 5 min at 200 C. These spectra were recorded 1 h after a switch from the model gas mixture (CO2/H2 [ 1/9) to 5% H2/ Ar.
Conclusions The effect of CeO2 loading amount on CO2 methanation activity and selectivity was investigated using Ru/Al2O3, Ru/30%CeO2/ Al2O3, Ru/60%CeO2/Al2O3, and Ru/CeO2. The order of CO2 reac C was Ru/30%CeO2/Al2O3 tion rate at 250 1 1 (6.2 ml min1 g1 ) > Ru/60%CeO gcat) > Ru/ cat 2/Al2O3 (4.9 ml min 1 1 CeO2 (4.6 ml min1 g1 ) [ Ru/Al O (1.2 ml min g cat 2 3 cat), which means that Ru/CeO2/Al2O3 catalysts showed the high activity of CO2 methanation. Furthermore, CH4 selectivity of Ru/CeO2/ Al2O3 was higher than that of Ru/CeO2. The Ru/CeO2/Al2O3 catalysts developed in this study exhibited the following four features suitable for CO2 methanation catalysts. (i) Transmission electron microscope observation revealed that the average particle sizes of CeO2 on Ru/ 30%CeO2/Al2O3 (6.5 nm) and Ru/60%CeO2/Al2O3 (10 nm) were smaller than that on Ru/CeO2 (13 nm), indicating that CeO2 on Ru/CeO2/Al2O3 was highly dispersed on Al2O3. (ii) It was confirmed by temperature programmed reduction by H2 that partial surface reduction of CeO2 on Ru/ CeO2/Al2O3 was promoted compared to Ru/CeO2, resulting in the enhancement of CO2 methanation. (iii) Fourier transform infrared spectroscopy measurements demonstrated that decomposition of formate species, known as intermediates of CO2 methanation, was fast over Ru/CeO2 and Ru/CeO2/Al2O3, in contrast of Ru/ Al2O3, leading to improvement of CO2 reduction to CO. (iv) Superior CO methanation activity over Ru/CeO2/Al2O3 led to the high CH4 selectivity close to 1.
Acknowledgments TEM observation was conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the Ministry of Education, Culture, Science, and Technology, Japan.
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Promotion of CO2 methanation activity and CH4 selectivity at low temperatures over Ru/CeO2/Al2O3 catalysts Shohei Tada a,b, Ochieng James Ochieng a,1, Ryuji Kikuchi a,*, Takahide Haneda c, Hiromichi Kameyama c a
Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Japan Society for the Promotion of Science, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 103-0083, Japan c Tokyo Gas Co., Ltd., 1-7-7 Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa 230-0045, Japan
article info
abstract
Article history:
The effect of CeO2 loading amount of Ru/CeO2/Al2O3 on CO2 methanation activity and CH4
Received 27 December 2013
selectivity was studied. The CO2 reaction rate was increased by adding CeO2 to Ru/Al2O3,
Received in revised form
and the order of CO2 reaction rate at 250 C is Ru/30%CeO2/Al2O3 > Ru/60%CeO2/Al2O3 > Ru/
17 April 2014
CeO2 > Ru/Al2O3. With a decrease in CeO2 loading of Ru/CeO2/Al2O3 from 98% to 30%,
Accepted 20 April 2014
partial reduction of CeO2 surface was promoted and the specific surface area was enlarged.
Available online 16 May 2014
Furthermore, it was observed using FTIR technique that intermediates of CO2 methanation, such as formate and carbonate species, reacted with H2 faster over Ru/30%CeO2/Al2O3 and
Keywords:
Ru/CeO2 than over Ru/Al2O3. These could result in the high CO2 reaction rate over CeO2-
Ruthenium
containing catalysts. As for the selectivity to CH4, Ru/30%CeO2/Al2O3 exhibited high CH4
Hydrogen transfer
selectivity compared with Ru/CeO2, due to prompt CO conversion into CH4 over Ru/30%
Ceria
CeO2/Al2O3.
CO2 methanation
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Utilization of green hydrogen, produced by renewable energy (such as biomass, solar, and wind energy), has been urged worldwide in order to decrease the reliance on fossil fuel, to minimize the environmental pollution, and to produce hydrogen sustainably [1]. Fuel cells have been developed as a
promising power generator with high efficiency and low environmental impact. Fuel cells directly convert chemical energy of hydrogen into electrical energy and thus avoid the intermediate steps of producing heat and mechanical power works typical of conventional power generation methods. Thus the efficiency of the energy conversion from fuels is not limited by thermodynamic limitations of heat engines such as Carnot efficiency. In addition, the fuel cells can produce
* Corresponding author. Tel./fax: þ81 3 5841 1167. E-mail addresses: [email protected], [email protected] (R. Kikuchi). 1
Present address: Department of Environment Systems, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan. http://dx.doi.org/10.1016/j.ijhydene.2014.04.133 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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electricity (and heat) with low pollutant emissions due to the avoidance of combustion. Hydrogen is a gas at standard temperature and pressure, and has many difficulties in storing and transporting hydrogen. In order to overcome the difficulties, many studies have been conducted on the following hydrogen carriers: CH4 [2e5], NH3 [6], MeOH [5,6], dimethyl ether [5,7], formic acid [5,8], and organic chemical hydrides [1,9,10]. Especially CH4, made from CO2 methanation (CO2 þ 4H2 / CH4 þ 2H2O), has become a promising candidate as hydrogen source to be used in fuel processors. The advantage of CH4 as a hydrogen carrier is non-toxicity in contrast to ammonia and methanol, and has a high energy density (3 times higher volumetric energy density than hydrogen). Moreover the well-developed infrastructures of natural gas can be adopted for methane, resulting in the easy storage and transportation of methane [6]. Added to this, enormous carbon dioxide emitted into atmosphere can be recycled as the feedstock of CO2 methanation, leading to the solutions to global warming [2]. If the cost of green hydrogen production can be lowered, this attempt will succeed admirably. CO is one of the intermediates in CO2 methanation and produced in the following reactions: reverse water gas shift reaction (RWGS reaction, CO2 þ H2 / CO þ H2O) at more than 200 C [11e13] and CO2 decomposition (CO2ads / COads þ Oads) at less than 100 C [3,14,15]. The CO2 reduction to CO is considered to be a ratedetermining step of CO2 methanation. Endothermic RWGS reaction is favored thermodynamically and kinetically at high temperatures. The successive CO methanation (CO þ 3H2 / CH4 þ H2O) is, however, of exothermic nature, and thus lower reaction temperature is desirable for higher CH4 selectivity. It is, therefore, important to enhance the reaction rate of RWGS reaction and CO methanation at low temperatures. The mechanism of CO2 methanation has been mainly investigated using Ni [2,11,16e18] and noble metal (such as Ru [12], Rh [3,14,15], and Pd [13]) as active species supported on various metal oxides. As for carbon dioxide methanation over supported metal catalysts, it is well accepted that carbon dioxide adsorbed on the catalyst surface reacts with hydrogen on the metal to produce methane [11,16,19]. However, there still exists argument that the active sites of the conversion from CO2 to CO over supported catalysts are the metal surface [17,20,21] or interface between the metal and the support [12,22e24]. The reported catalysts for CO2 methanation were summarized in Table 1. Sharma et al. reported that over Ru-doped ceria, Ce0.95Ru0.05O2, prepared by a combustion method, CO is not a reaction intermediate of CO2 methanation [25]. The catalyst showed higher catalytic activity for CO2 methanation
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than 5 wt% Ru/CeO2, and CO2 conversion and CH4 selectivity were 55% and 99%, respectively, at 450 C by feeding 13% CO2, 54% H2, and 33% Ar at GHSV ¼ ca. 10,000 h1. In addition, CH4 was not produced up to 550 C when CO and H2 were fed to the catalyst. Ocampo et al. investigated CO2 methanation over Ni/ Ce0.72Zr0.28O2 catalysts [19]. Nickel sintering on this catalysts was restricted by the incorporation of nickel cations into the fluorite-structured Ce0.72Zr0.28O2, resulting in high stability for 150 h at 350 C during CO2 methanation (H2/CO2/N2 ¼ 36/9/10, GHSV ¼ 43,000 h1). In our previous work, the catalytic performance of Ni/CeO2, Ni/Al2O3, Ni/MgO, and Ni/TiO2 was investigated for CO2 methanation [26]. It is concluded that CeO2 plays an important role in promoting CO2 methanation and enhancing CH4 selectivity. Sufficient amount of CO2 adsorbed on CeO2 which is known as a basic oxide, and CO2 was reduced easily due to the oxygen vacancies on CeO2 support, leading to high CO2 conversion at low temperatures below 350 C. In addition, methanation of CO, which is the intermediate of CO2 methanation, was improved over Ni/ CeO2, resulting in high CH4 selectivity at low temperatures. Over CeO2-containing catalysts, CeO2 surface with surface oxygen vacancies influences on CO2 methanation activity. Thus an effective utilization of CeO2 by enlarging the surface area is anticipated to improve the CO2 methanation. We focused on ceria-supported materials as support material, and tried to increase CeO2 surface area. The ceria-supported materials have been reported in the following fields: cellulose gasification (Rh/CeO2/SiO2) [27], HCl oxidation to Cl2 (CeO2/ ZrO2) [28], oxidation of 1,2-dichloroethane (CeO2/USY zeolite) [29], and CO methanation (CeO2-doped Ni/Al2O3) [30]. Rynkowski et al. studied Ru/CeO2eAl2O3 catalysts for CO2 methanation, and observed CeAlO3 formed by high temperature reduction pretreatment (above 973 K) [31]. They expected that the stabilization of Ce3þ in CeAlO3 leads to high activity of CO2 methanation. Trovarelli et al. improved CeO2 reducibility on Rh/CeO2/SiO2 due to deposition of CeO2 on silica and H2 spillover from Rh to CeO2, leading to high CO2 methanation activity [32]. Therefore, these two groups suggested the importance of partially-reduced CeO2 surface on CO2 methanation. In this study, Ru species was loaded on CeO2/Al2O3, CeO2, and Al2O3 by an impregnation method, and the effect of CeO2 loading amounts on CO2 methanation activity and CH4 selectivity was investigated using these supported Ru catalysts. Expansion of CeO2 surface area by dispersing CeO2 on Al2O3 is expected to increase (i) CO2 adsorption per unit catalyst weight on the CeO2 catalysts, and (ii) the number of utilizable oxygen vacancies, resulting in high CO2 conversion at low temperatures. Furthermore CeO2 supported metal catalysts are anticipated to show high CO methanation activity, leading to high CH4 selectivity at low temperatures.
Table 1 e Reported CO2 methanation catalysts. Catalysts
Gas composition
Grac¸a et al. Ocumpo et al.
14%Ni7%CeUSY 5%Ni/Ce0.72Zr0.28O2
CO2/H2/N2 ¼ 9/36/10 CO2/H2/N2 ¼ 9/36/10
Sharma et al. Tada et al.
Ce0.95Ru0.05O2 10%Ni/CeO2
CO2/H2/Ar ¼ 13/54/33 CO2/H2 ¼ 1/4
GHSV/h1 43,000 21,000 43,000 10,000 10,000
CO2 conversion
68%, 400 C ca. 80%, 350 C ca. 65%, 350 C 55%, 450 C 87%, 300 C
Reference [18] [19] [25] [26]
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Experimental methods Catalyst preparation Chemicals Chemicals used in the synthesis were Al2O3 (JRC-ALO-6) provided by Catalysis Society of Japan, cerium (III) nitrate hexahydrate (Ce(NO3)3$6H2O, Aldrich, 98%), and ruthenium nitrate solution (Ru(NO3)3, Tanaka Kikinzoku Kogyo K.K., 3.920 wt%).
Preparation of CeO2/Al2O3 and CeO2 CeO2/Al2O3 was prepared by an impregnation method. Al2O3 was impregnated with a 100 ml aqueous solution of Ce(NO3)3$6H2O. The samples were dried at 100 C and then calcined at 500 C for 3 h in air. The CeO2 loadings were 30 and 60 wt%. Pure CeO2 was also made in the following manner: drying of Ce(NO3)3$6H2O solution at 100 C, and calcination at 500 C for 3 h in air. The samples of Al2O3, 30%CeO2/Al2O3, 60%CeO2/Al2O3, and CeO2 were hereafter abbreviated as AL, 30CEAL, 60CEAL, and CE, respectively.
Preparation of supported Ru catalysts Supported Ru catalysts were prepared by an impregnation method. The AL, 30CEAL, 60CEAL, CE were impregnated with a 100 ml aqueous solution of Ru(NO3)3. All samples were dried at 100 C and then calcined at 500 C for 3 h in air. The Ru loading was 2 wt%.
Characterization The specific surface area of each catalyst was evaluated by the BET method using nitrogen adsorption (Micromeritics, ASAP2000). Before the measurement, all samples were dried in vacuum at 150 C. Transmission electron microscopy (TEM) monitored the morphology of the CeO2 and Al2O3 particles on the catalysts (JEOL, JEM-2000EX, acceleration voltage 200 kV). The crystalline phase of catalysts was determined by X-ray diffraction (XRD) using Rigaku, RINT 2400 (voltage 40 kV; current 100 mA). X-ray photoelectron (XP) spectra were measured by a JEOL JPS-90SX with MgKa radiation. The binding energy for each XP spectroscopy measurement was referenced to the C 1s peak (285.0 eV). Temperature programmed desorption of CO2 (CO2-TPD) was carried out in a flow system (Quanta Chrome, CHEMBET-3000). About 100 mg of each catalyst was flushed at 500 C for 30 min in He flow and cooled to 100 C in He flow. A gaseous mixture of 10% CO2/He was fed to the reactor at 100 C for 30 min, and the temperature was raised at a constant heating rate of 10 C min1 under He flow. The reducibility of ruthenium and cerium was evaluated by temperature programmed reduction (TPR) in a H2 flow system (Quanta Chrome, CHEMBET-3000). In a gas stream of 5%H2/Ar, about 20 mg of the samples was heated up to 400 C at 10 C min1. Before the TPR measurement, each sample was flushed with He flow at 500 C for 30 min and cooled to room temperature. Ru and Ce loading amounts on each catalyst were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, SII NanoTechnology, SPS4000).
Fourier transform infrared spectroscopy measurement A Fourier transform infrared spectroscopy (FTIR) cell equipped with high purity CaF2 windows and capability for heating and cooling was placed in a JASCO FTIR 6100 instrument with a mercury cadmium telluride (MCT) detector (JASCO, MCT6000M). The powdered samples were pressed into a thin self-supporting disk and set in the cell. Typically, 50 scans were collected for one spectrum, and the results are presented as absorbance spectra. The following two experiments were carried out.
Temperature programmed desorption of CO2 with FTIR Adsorption and desorption characteristics of CO2 on Ru/CeO2/ Al2O3 were investigated. The disk was flushed with N2 stream at 300 C as a pretreatment. Temperature was then decreased in a stepwise mode to room temperature and background spectra were attained at desired temperatures in interest after equilibration for 10 min-on-stream. The sample was exposed to the mixed gas (10%CO2/He) for 10 min at room temperature. The cell was purged with N2 and their spectra were recorded at the desired temperatures during heating (heating rate ¼ 5 C/min).
Reaction intermediate observation during CO2 methanation on CeO2-containg catalysts After reduction by 5%H2/Ar at 350 C for 30 min in the cell, the samples were held at 200 C in a N2 stream. Then background spectra were collected at 200 C. The samples were exposed to the reaction gas (CO2/H2 ¼ 1/9) for 5 min at 200 C, and then their spectra were recorded 10 min after the mixed gas had been removed by flushing with N2. Subsequently, the behavior of products adsorbed on the catalyst surface was also examined in N2 and H2/Ar gas. After 5 min reaction, the samples were exposed to N2 or 5%H2/Ar at 200 C, and then their spectra were measured.
Activity tests Catalytic performance of CO2 methanation was evaluated in a 4mm I.D. fixed-bed quartz tubular reactor at atmospheric pressure. The reaction temperature was measured at the inlet of the catalyst bed by a K-type thermocouple. Catalyst powder of 300 mg was set in the reactor, and then reduced at 500 C for 30 min in 5% H2/Ar flow prior to the run. A gaseous mixture of 20% CO2/H2 was fed at a space velocity of 10,000 h1. Catalytic activity of CO methanation was also measured in the same apparatus by using a sample weight (300 mg) and a reaction gas condition (1%CO/H2, 10,000 h1). The gas composition at the reactor outlet was analyzed with a micro gas chromatograph (Varian, CP-4900) equipped with MS5A, COX, and PPQ columns and a thermal conductivity detector (TCD). The outlet gas concentrations were described in a basis of the dry gas composition.
Results and discussion Effect of CeO2 loading on catalytic activity over Ru/CeO2/ Al2O3 Fig. 1(a) shows CO2 conversion over the supported Ru catalysts as a function of reaction temperature. CO2 conversion, CH4 selectivity, and CO2 reaction rate are defined as follows:
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upper shift of the measured CO2 conversion to equilibrium one in Fig. 1(a). In Fig. 1(b) the CH4 selectivity over the supported Ru catalysts is plotted against reaction temperature. The selectivities of Ru/AL, Ru/30CEAL, and Ru/60CEAL were close to 1 at low temperatures below 450 C. The selectivity of Ru/CE was 0.986 at 250 C, and ca. 1 at temperatures between 300 and 450 C. The rest of converted CO2 existed as CO, which means that the improvement of CO methanation activity leads to high CH4 selectivity in CO2 methanation. As for Ru/ 30CEAL and Ru/60CEAL CO methanation activity was expected to be high compared to Ru/CE because of the high CO2 reaction rates and CH4 selectivities. The conversion curves of CO during CO methanation over the supported Ru catalysts were shown in Fig. 2. The concentration of CO over Ru/30CEAL increased rapidly compared to that over Ru/CE, which indicates that Ru/30CEAL showed higher CO methanation activity than Ru/CE, and thus Ru/ 30CEAL was concluded to have a high CH4 selectivity during CO2 methanation.
Characterization XRD Fig. 1 e (a) CO2 conversion and (b) CH4 selectivity over supported Ru catalysts. Reaction conditions: 20 vol% CO2, 80 vol% H2. GHSV [ 10,000 hL1. The temperature was measured at the catalyst bed inlet.
FCO2 ;out FCO2 ;in
(1)
FCH4 ;out FCO2 ;in FCO2 ;out
(2)
CO2 conversion ¼ 1
CH4 selectivity ¼
CO2 reaction rate ¼
CO2 conversion FCO2 ;in W
(3)
where FCO2 ; in and FCO2 ; out are molar flow rates of CO2 at the reactor inlet and outlet, respectively, FCH4 ; out is the outlet molar flow rate of CH4, and W is the sample weight. Over Ru/ AL CO2 conversion was gradually increased as the temperature was raised, and at 500 C reached the equilibrium one represented by the solid line in Fig. 1(a) calculated by taking CO2 methanation and RWGS reaction into account. Over the other supported Ru catalysts the conversion was rapidly increased consistently with increasing temperature up to 350 C, and then achieved the equilibrium one. The reaction rates at 250 C over Ru/AL, Ru/30CEAL, Ru/60CEAL, and Ru/CE were 1.2, 6.2, 4.9, and 4.6 ml min1 g1 cat, respectively, which indicates that Ru/CeO2/Al2O3 catalysts (especially Ru/30CEAL) have higher CO2 methanation activity than Ru/Al2O3 and Ru/ CeO2. As for CeO2-containing catalysts, CO2 conversion above 400 C was slightly above the equilibrium one probably due to temperature gradient in the catalyst bed. Actually, CO2 conversion at 450 C was in the order of the length of the catalyst bed: Ru/30CEAL (31 mm) > Ru/60CEAL (22 mm) > Ru/CE (16 mm). According to the gradient, part of CO2 was methanated at the temperature measured by K-type thermocouple, and the rest at lower temperature, probably leading to the
To investigate the crystalline phases of supported Ru catalysts, powder XRD measurements of AL, CE, Ru/AL, Ru/ 30CEAL, Ru/60CEAL, and Ru/CE were performed, and all peaks were identified with JCPDS files, as illustrated in Fig. 3. Both AL and CEAL catalysts exhibited similar diffraction patterns with peaks at 38, 39, 46, and 67 , which correspond to (311), (222), (400), and (440) planes of g-Al2O3, respectively. The supported Ru catalysts have 4 peaks at 28, 35, 54, and 58 in the XRD patterns, which correspond to (110), (101), (211), and (220) planes of RuO2, respectively. Both CE and CEAL catalysts showed the patterns with the peaks at 29, 33, 47, 56, 59, and 69 , which correspond to (111), (200), (220), (311), (222), and (400) planes of fluorite-structured CeO2, respectively. The mean crystallite size of RuO2 and CeO2 was estimated from the diffraction peak of RuO2 (101) and CeO2 (111) plane by Scherrer equation, D¼
Kl b cos q
(4)
where K is the shape factor (0.89), l is X-ray wavelength (0.154 nm), b is the line broadening at half the maximum
Fig. 2 e CO conversion over Ru/CE and Ru/30CEAL. Reaction conditions: 1 vol% CO, 99 vol% H2. GHSV [ 10,000 hL1.
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Fig. 3 e XRD patterns of as-prepared AL, Ru/AL, Ru/30CEAL, Ru/60CEAL, Ru/CE, and CE. JCPDS-card number: fluoritestructured CeO2 (71-4199), g-Al2O3 (29-0063), and RuO2 (40-1290).
intensity in radians, and q is Bragg angle [33]. The sizes of RuO2 on Al2O3, 60%CeO2/Al2O3, and CeO2, as summarized in Table 2, were ca. 30 nm, and that on 30%CeO2/Al2O3 was 16 nm. The sizes of CeO2 were almost constant (ca. 8 nm).
Fig. 4(d) exhibits that as for 30CEAL small CeO2 particles (average 6.5 nm) were dispersed on rod-shaped g-Al2O3, and the formation of the CeO2 secondary particles (ca. 50 nm, indicated as solid arrows) can be observed.
Electron microscopic image
Reducibility and loading amount of supported Ru catalysts measured by ICP and H2-TPR
As summarized in Fig. 4, transmission electron microscopy (TEM) was used to examine the AL, CE, 60CEAL, and 30CEAL. Fig. 4(a) displays that the shape of g-Al2O3 was rod, and the diameter was ca. 3 nm. Fig. 4(b) shows a lot of CeO2 particles with average size of 13 nm. On the CeO2 particles, slightly bright dots can be seen, which means that this CeO2 had a porous structure. Fig. 4(c) shows that 60CEAL is composed of a lot of CeO2 particles and small amount of rod-like g-Al2O3 (indicated as solid arrows). The average particle size of CeO2 on 60CEAL was 10 nm and a little smaller than that on CE.
Fig. 5 shows the H2-TPR profiles of the supported Ru catalysts. Over the 4 catalysts, reduction peaks were detected below 300 C, which have been ascribed to reduction of RuO2 [34e36]. H2 consumption was calculated by assuming reduction of Ru4þ (RuO2) to Ru0, as summarized in Table 3. As for Ru/AL, the consumption amount estimated by H2-TPR 1 was 0.29 mmol g1 cat. This value is close to 0.30 mmol gcat which accords with the H2 consumption by RuO2 reduction to 1.5 wt% Ru metal. Thus Ru oxides on Ru/AL was completely
Table 2 e Characterization results for support materials and supported Ru catalysts. Catalysts
CE þRu 60CEAL þRu 30CEAL þRu AL þRu a b
SSA/m2 g1
69 73 81 74 104 115 164 162
Pore volume/cm3 g1
0.20 0.18 0.25 0.22 0.41 0.42 0.66 0.62
Crystallite sizea/nm RuO2(101)
CeO2(111)
e 28 e 28 e 16 e 30
9.6 7.8 8.2 7.9 8.5 8.4 e e
Estimated from XRD patterns. CO2 adsorption per unit specific surface area. Estimated from CO2-TPD and N2 adsorption.
CO2 adsorptionb/(mmol g1)/(m2 g1)
0.72 1.40 0.40 0.32
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Fig. 4 e TEM images of (a) AL, (b) CE, (c) 60CEAL, and (d) 30CEAL. Particle size distributions of CeO2 in (e) CE, (f) 60CEAL, and (g) 30CEAL measured by the TEM observation.
reduced. On the other hand, the amounts of Ru on Ru/CE, Ru/ 60CEAL, and Ru/30CEAL by H2-TPR (0.67e0.77 mmol g1 cat) are almost twice as much those estimated by ICP-AES (0.32e0.35), which means that not only RuO2 but also a part of CeO2 was reduced below 300 C. Indeed CeO2 alone can be reduced, as evidenced by the broad peak at 470 C on the TPR profile [26], and the CeO2 reducibility was enhanced due to the presence of Ru. As for supported metal catalysts a part of H2 molecules dissociated on noble metals such as Ru may migrate to nearby metal oxides under a H2-rich condition. This phenomenon is known as a hydrogen spillover effect [13,36,37]. Thus it is expected that over our Ru/CE, Ru/60CEAL, and Ru/30CEAL CeO2 in the vicinity of Ru species was selectively reduced by spillover hydrogen. Interestingly, H2 consumption by CeO2 reduction per unit CeO2 weight was increased with decreasing CeO2 loadings, as listed in Table 3. This result indicates that the CeO2 particle size became smaller for the decrease in the CeO2 loadings, leading to enlarged utilizable CeO2 surface area.
Surface analysis of supported Ru catalyst BET surface areas and pore volumes of AL, 30CEAL, 60CEAL, and CE determined by N2 adsorption are listed in Table 2. The
Fig. 5 e H2-TPR profiles of Ru/AL, Ru/30CEAL, Ru/60CEAL, and Ru/CE (heating rate [ 10 C/min).
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Table 3 e Loading weight and H2 consumption. Catalysts
Ru/CE Ru/60CEAL Ru/30CEAL Ru/AL a b c
Loading weighta/wt%
H2 consumption/mmol g1 cat
Ru
CeO2
RuO2 reduction
1.8 1.8 1.6 1.5
98 52 26 e
0.35 0.35 0.32 0.30
a
H2 consumption by CeO2 reductionc/mmol gCeO2 1
b
Total 0.77 0.65 0.67 0.29
0.43 0.58 1.34 e
Calculated by ICP-AES. Estimated by the peak areas of H2-TPR spectra. The H2 consumption difference between total and RuO2 reduction per unit CeO2 weight.
surface area decreased from 164 to 69 m2 g1 with increase in CeO2 loading amounts, and the pore volume simultaneously dropped from 0.66 to 0.20 cm3 g1. Especially both the surface area and the pore volume of 60CEAL were similar to those of CE. This is because CeO2 plugged the surface and pores of Al2O3. The addition of Ru species to the supports did not change the surface area and the pore volume, indicating that the Ru components did not cause sintering and pore blockage of the support of the supports. XP spectra of Al 2p core electron levels for as-prepared Ru/ CE, Ru/60CEAL, Ru/30CEAL, and Ru/AL are shown in Fig. 6(a). The spectra from Ru/60CEAL, Ru/30CEAL, and Ru/AL have a single peak at 74 eV. This peak is attributed to Al3þ in g-Al2O3 [38]. The order of the intensity was Ru/AL S Ru/30CEAL > Ru/ 60CEAL, and there was no peaks of Al 2p over Ru/CE. Fig. 6(b) indicates O 1s spectra of the catalysts. The spectrum of Ru/CE shows a strong peak at 529 eV with a small shoulder at 531 eV. The first peak in the spectrum is attributable to a lattice oxygen in CeO2 and the second at 531 eV to a defect-oxygen and a
hydroxyl-like group in CeO2 [29,38,39]. The O 1s spectrum of Ru/60CEAL was split up into two peaks at 531 and 529 eV, which are assigned to lattice oxygen of Al2O3 and CeO2, respectively. As for Ru/30CEAL, the O 1s peak of Al2O3 with the binding energy at 531 eV was solely observed. In the Ru 3d region shown in Fig. 6(c), the peak at 281 eV appeared and belongs to Ru4þ in RuO2 [40]. The Ce 3d spectra for supported Ru catalysts are shown in Fig. 6(d). Six peaks in the spectra correspond to Ce4þ in CeO2 [38,39,41]. In order to discuss the amount of exposed Ce species, the surface atomic ratio RS is calculated from the following equation. RS ¼
ICe =SCe ICe =SCe þ IAl =SAl
(5)
, where Ix the intensity of peaks assignable to an element x in XP spectra, and Sx the atomic sensitive factor of x [42]. The molar ratio of Ce to Al on Ru/CeO2, Ru/Al2O3, and Ru/CeO2/ Al2O3 catalysts are summarized in Table 4. The bulk
Fig. 6 e XP spectra of (a) Al 2p, (b) O 1s, (c) Ru 3d, and (d) Ce 3d of supported Ru catalysts.
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Table 4 e Bulk and surface molar ratio of Ce and Al estimated from ICP-AES and XP spectra. Catalysts
Ru/CE Ru/60CEAL Ru/30CEAL Ru/AL a b
Molar ratio Ce/(Ce þ Al) Bulk RBa
Surface RSb
1 0.25 0.10 0
1 0.36 0.06 0
Calculated from ICP-AES results listed in Table 2. Estimated from the area of peaks attributed to Ce and Al.
composition ratios (RB) were initially approximated by the ICPAES results. With respect to Ru/30CEAL, RB was higher than RS, indicating that CeO2 tends to infiltrate into Al2O3 pore. This interpretation is also supported by N2 adsorption results described above e CeO2 particles sealed a part of Al2O3 pores. Concerning Ru/60CEAL, RB was lower than RS, which means that Al2O3 surface was covered with CeO2. In our previous study, the surface coverage by CO2-derived species, such as carbonate and formate species, on catalysts resulted in the high CO2 methanation activity [26]. Thus it is necessary to investigate CO2 adsorption on supported Ru catalysts. Fig. 7(a) shows a series of FTIR spectra of CO2 adsorbed on Ru/30CEAL recorded with increasing temperature. In the spectrum at 24 C, bands at 1650 and 1230 cm1 appeared, and were attributable to bidentate carbonate species [43]. Furthermore two peaks at 1570 and 1430 cm1 were assignable to unidentate carbonate species [43]. The intensity of the 4 peaks was decreased with temperature elevation from
Fig. 7 e (a) FTIR spectra of CO2 adsorbed on Ru/30CEAL in the course of desorption at elevated temperatures from 25 C to 300 C (heating rate: 5 C/min). (b) CO2-TPD profiles of Ru/AL, Ru/30CEAL, Ru/60CEAL, and Ru/CE (heating rate [ 10 C/min).
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24 C to 200 C, and then no peaks could be observed in the region between 2200 and 1100 cm1, which shows that CO2 was completely desorbed from Ru/30CEAL at 200 C. The TPD spectra obtained after CO2 adsorption on the supported Ru catalysts at 100 C are shown in Fig. 7(b). As for all catalysts, two desorption peaks were detected at around 110 C and 180 C. These are assignable to CO2 desorption because the peaks of carbonate species in FTIR spectra disappeared at the temperatures above 200 C (Fig. 7(a)). The amount of CO2 adsorption per unit catalyst weight was calculated by the peak area of CO2-TPD and BET surface area, as listed in Table 2. The order of the adsorption was Ru/ 60CEAL > Ru/CE > Ru/30CEAL > Ru/AL, indicating that a large amount of CO2 adsorbed on CeO2/Al2O3 supported Ru catalysts, especially Ru/60CEAL, compared with Ru/AL.
Reaction intermediates measured by FTIR analysis Fig. 8 shows the time variation of FTIR spectra at 200 C by feeding a model gas mixture of CO2/H2 ¼ 1/9 for CO2 methanation. The Ru/AL, Ru/30CEAL, and Ru/CE catalysts were reduced prior to the reaction. As for Ru/AL in Fig. 8(a), five distinct peaks at 1980, 1590, 1450, 1390, and 1370 cm1 became stronger as reaction time elapsed from 10 s to 5 min. For Ru/
Fig. 8 e FTIR spectra of (a) Ru/AL, (b) Ru/30CEAL, and (c) Ru/ CE during CO2 methanation with the model gas mixture (CO2/H2 [ 1/9) at 200 C. The spectra were obtained in the following manner: reaction for desired time, and flushing 10 min with N2.
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30CEAL in Fig. 8(b), the peaks at 2020, 1940, 1590, 1520, 1470, 1450, 1390, 1370, and 1290 cm1 arose during CO2 methanation reaction. On Ru/CE, right after the introduction of 10%CO2/H2 gas mixture into the IR cell, eight peaks at 2020, 1940, 1580, 1530, 1470, 1370, 1330, and 1290 cm1 were detected as shown in Fig. 8(c). On all catalysts, the bands at the wavenumbers between 2020 and 1940 cm1 were attributed to CO adsorbed linearly on Ru surface while the other bands to carbonate or formate species on the catalysts [24,44]. In order to assess the reaction steps in which these species are involved, transient experiments in H2/Ar atmosphere were undertaken. As shown in Fig. 9(a), (b), and (c), the bands at ca. 2000 cm1, attributed to CO adsorbed on Ru species, were diminished. Over Ru/CE, especially, a new peak at 1820 cm1 appeared with an increase in retention time under H2/Ar atmosphere, which is assigned to bridge-bonded CO on Ru [24]. As for Ru/AL in Fig. 9(a), the peaks at 1590, 1390, and 1370 cm1 were ascribed to formate species on Al2O3 [45]. In addition, the carbonate peak at 1450 cm1 was also unvaried. On Ru/CE (Fig. 9(c)), all peaks in the region 1600e1200 cm1 were weakened. To compare the area decreasing rate of each peak over Ru/CE, the spectra in Fig. 9(c) were divided into 6 peaks. The peak areas after desired time of reaction with H2/Ar were
Fig. 9 e FTIR spectra of (a) Ru/AL, (b) Ru/30CEAL, and (c) Ru/ CE after CO2 methanation for 5 min at 200 C. These spectra were recorded after a switch from the model gas mixture (CO2/H2 [ 1/9) to 5% H2/Ar.
defined as A, and the initial areas as A0. Fig. 10 shows the peak area ratios (A/A0) as a function of time-on-stream after switching to H2/Ar. The bands at 1580 and 1330 cm1 decreased at an identical rate, and are derived from disappearance of bidentate carbonate species on CeO2 [46,47]. Interestingly, the three curves for A/A0 at 1530, 1470, and 1370 cm1 were nearly coincident: the ratios of 1530, 1470, and 1370 cm1 decreased from 1 to 0.5 in 150 min. The peaks at 1530, and 1370 cm1 are ascribed to OeCeO asymmetric vibration and CeH in-plane bending vibration in formate species, respectively, while the band at 1470 cm1 assigned to monodentate carbonate [46]. Fig. 11 shows FTIR spectra of Ru/ CE recorded 1 h after a switch from the model gas mixture for CO2 methanation to N2. Under N2 atmosphere the carbonate peaks (1580 and 1330 cm1) gradually disappeared, while the formate peaks (1540 and 1390 cm1) were unchanged. Thus, what follows these results are: (i) the carbonate and formate species react with H2, (ii) the carbonate species on Ru/CE are decomposed or desorbed at high temperature such as 200 C, and (iii) the formate species on Ru/CE are thermally stable at 200 C. Over Ru/30CEAL, the intensities of the peaks attributed to RueCO (2020 and 1940 cm1), carbonate species (1330 cm1), and formate species (1590, 1390, 1370 cm1) were decreased with increasing reaction time under H2/Ar, as shown in Fig. 9(b). The peaks of CO adsorbed on Ru were rapidly diminished compared to Ru/CE. From this result and CO methanation activity test (Fig. 2), Ru/30CEAL exhibited high CO methanation activity, and thus high CH4 selectivity was attained in CO2 methanation. Interestingly the peak at 1390 cm1, assignable to formate species on Ru/Al2O3, remained almost unchanged even under H2/Ar flow. The peaks attributed to formate species on Ru/30CEAL and Ru/CE were weakened rapidly compared to Ru/AL, which indicates that the decomposition of formate species on CeO2-containg catalysts was promoted.
Fig. 10 e Decrease in FTIR peak area ratio to the initial peak area at the respective wavenumbers under 5% H2/Ar flow at 200 C as a function of reaction time.
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references
Fig. 11 e FTIR spectra of Ru/CE after CO2 methanation for 5 min at 200 C. These spectra were recorded 1 h after a switch from the model gas mixture (CO2/H2 [ 1/9) to 5% H2/ Ar.
Conclusions The effect of CeO2 loading amount on CO2 methanation activity and selectivity was investigated using Ru/Al2O3, Ru/30%CeO2/ Al2O3, Ru/60%CeO2/Al2O3, and Ru/CeO2. The order of CO2 reac C was Ru/30%CeO2/Al2O3 tion rate at 250 1 1 (6.2 ml min1 g1 ) > Ru/60%CeO gcat) > Ru/ cat 2/Al2O3 (4.9 ml min 1 1 CeO2 (4.6 ml min1 g1 ) [ Ru/Al O (1.2 ml min g cat 2 3 cat), which means that Ru/CeO2/Al2O3 catalysts showed the high activity of CO2 methanation. Furthermore, CH4 selectivity of Ru/CeO2/ Al2O3 was higher than that of Ru/CeO2. The Ru/CeO2/Al2O3 catalysts developed in this study exhibited the following four features suitable for CO2 methanation catalysts. (i) Transmission electron microscope observation revealed that the average particle sizes of CeO2 on Ru/ 30%CeO2/Al2O3 (6.5 nm) and Ru/60%CeO2/Al2O3 (10 nm) were smaller than that on Ru/CeO2 (13 nm), indicating that CeO2 on Ru/CeO2/Al2O3 was highly dispersed on Al2O3. (ii) It was confirmed by temperature programmed reduction by H2 that partial surface reduction of CeO2 on Ru/ CeO2/Al2O3 was promoted compared to Ru/CeO2, resulting in the enhancement of CO2 methanation. (iii) Fourier transform infrared spectroscopy measurements demonstrated that decomposition of formate species, known as intermediates of CO2 methanation, was fast over Ru/CeO2 and Ru/CeO2/Al2O3, in contrast of Ru/ Al2O3, leading to improvement of CO2 reduction to CO. (iv) Superior CO methanation activity over Ru/CeO2/Al2O3 led to the high CH4 selectivity close to 1.
Acknowledgments TEM observation was conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the Ministry of Education, Culture, Science, and Technology, Japan.
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